Asteroid vs Comet
Asteroids and comets are celestial bodies, which are smaller in size compared to the planets and their moons. They belong to the category of astronomical objects known as “Planetoids”.
What are Asteroids?
Asteroids are small, irregularly shaped, rocky celestial bodies in space, and they have the meaning “minor planets”. There are millions of Asteroids in space and the majority of the observed and known asteroids reside in orbits around the sun, located between Mars and Jupiter. This region is known as the Asteroid belt. Asteroids have elliptical orbits; i.e. they have low eccentricity, and the variation of the distance between the sun and the asteroid does not change largely. Orbital periods of the asteroids range from tens to hundreds of years.
Asteroids are believed to be the remnants from the early stages of planet formation, and most of the Asteroids in the asteroid belt are believed to be originated within the orbit of Jupiter. Mainly asteroids consist of solid material, such as metals and rocks, and they are inactive. They have irregular shapes as a result of the low mass of the body, which does not generate enough gravitational pull to acquire hydrostatic equilibrium before they solidify.
Sizes of the asteroids vary from hundreds of meters to hundreds of kilometers, but the majority (about 99%) of the asteroids has sizes below 1km. Largest asteroid known is Ceres which is located within the Asteroid belt.
What are Comets?
Comets are small icy bodies which produces a visible atmosphere when passing close to the sun. The heat from the sun turns the ices into gases and creates a gaseous shell called a coma around the body. Intense solar wind and radiation blows the atmosphere to create a tail pointing away from the sun. If the comets are in a visible range from the earth, it usually produces a spectacular scene in the night sky. For this reason comets are much widely known among the general public. In fact, comets were known to men before asteroids, because they were observable by the naked eye.
Most of the comets originate in the Kuiper Belt and in the Oort cloud, the regions in the outer rim of the solar system consisting of small icy bodies. When disturbed by an external force these icy bodies leave their low eccentric orbit around the sun and enter a highly elongated orbit with high eccentricity. When travelling through outer regions, these smaller bodies are inactive and accumulate material around them in the space.
In addition to nucleus, coma and tail, another feature can be observed on the surface of a comet. Surface of a comet in its inactive stages are rocky and are covered with dust accumulated from space. The ices are hidden beneath the surface about a meter below. Due to solar radiation the vaporized gasses exits the nucleus through fissures and cavities on the surface with high velocity to create visible gas jets. Majority of the material on a comet is water (H2O) ice, among frozen carbon dioxide (CO2), carbon monoxide (CO), and Methane (CH4). Organic compounds methanol, ethanol, ethane, and hydrogen cyanide can also be found on a comet in smaller amounts.
When the comet becomes active the surface activity increases and becomes volatile and the shape of the comet changes during this period.
Some comets are from outer space and have hyperbolic orbits. These comets travel through the solar system only once and catapulted ti interstellar space by the suns gravity ever to return. However, many of the comets resides inside the solar system in highly elongated elliptic orbits and comes near the sun periodically and becomes active. When moving away from the sun at the outer edges of the solar system, the nucleus replenishes its ice by accumulating the material in the colder surroundings. Though the accumulation is slower than the loss during the active stage, gradually the comet becomes dry and turns into an asteroid.
What is the difference between Asteroids and Comets?
• Asteroids mostly reside in the Asteroid belt located between the orbits of Mars and Jupiter. Comets mostly reside in the Kuiper Belt beyond the orbit of Neptune and in the Oort cloud of outer solar system.
• Asteroids are formed inside the orbit of the Jupiter while comets are formed in the outer edges of the solar system.
• Sizes of the asteroids vary from few centimeters to 900kms while the sizes of the comets range from 10 km to 50kms.
• Asteroids consist mainly of rocky and metal material while comets contain large amounts of frozen gases (water ice, carbon dioxide ice and carbon monoxide ice) along with hydrocarbons with a rocky structure.
• Comet surface is very unstable and changing when active, but asteroids surface is stable and steady with identifiable geography such as craters.
• Asteroids have no coma or tail while comets have both when near sun.
• Asteroids have low eccentricity elliptic orbits while comets have highly elongated elliptic orbits.